The Sling Index calculates slinging accuracy as margin of error in the angle of each shot. Assuming a circular target area. The point of release, the center of the target, and the edge of the target form a right-triangle. At the vertex where the slinger stands the angle gets smaller and smaller as the accuracy of the slinger improves since both increased distance and decreased target area shrink the angle. Therefore the lower the Sling Index, the smaller the angle of error possible to still allow a hit on the target, and therefore the better the accuracy.
I was asked recently how accurate I was at slinging and found the question difficult to answer. The size of the target, how many shots I took at it, and of course the all important, how far away I was, all depend on the likely hood of me hitting the target. I've slung with high and low percentages but the situations were not comparable. There must be a way to level the playing field, so that no matter what I'm slinging at and how far away I am, I can have a gauge for the level of accuracy I'm attaining to. To accomplish this from now on I'll be using the Sling Index.
Sling Index equals the inverse tangent of the target radius dividing by the distance of the target. So if the radius of your target is the same as your distance from the target, the sling index is 45. A very easy shot. At ten feet your target would be 300 square feet, about the size of a house. Conversely, if you are 19 feet from a 3 square foot target, the sling index is just over 3. A difficult shot, like standing on the sidelines of a football field and hitting a beach ball half way between you and the center of the field. This graph gives some idea as to how quickly the Sling Index drops with distance from a 1 square unit target.